Color helps us remember objects, influences our purchases and sparks our emotions. But did you know that objects do not possess color? They reflect wavelengths of light that are seen as color by the human brain.
The visible spectrum for humans falls between ultraviolet light and red light. Scientists estimate that humans can distinguish up to 10 million colors.
When light hits an object, such as a lemon, the object absorbs some of that light and reflects the rest of it. That reflected light enters the human eye first through the cornea, the outermost part of the eye. The cornea bends light toward the pupil, which controls the amount of light that hits the lens. The lens then focuses the light on the retina, the layer of nerve cells in the back of the eye.
Cones Influence Color Perception
Your retina has two different types of cells that detect and respond to light—rods and cones. These cells that are sensitive to light are called photoreceptors. Rods are activated when you’re in low or dim light. Cones are stimulated in brighter environments. Most of us have about 6 million cones, and 110 million rods.
Cones contain photo pigments, or color-detecting molecules. Humans typically have three types of photo pigments—red, green and blue. Each type of cone is sensitive to different wavelengths of visible light.
In the daytime, a lemon’s reflected light activates both red and green cones. The cones then send a signal along the optic nerve to the visual cortex of the brain. The brain processes the number of cones that were activated and the strength of their signal. After the nerve impulses are processed, you see a color— in this case, yellow.
In a darker environment, the light reflected by the lemon would stimulate only the eyes’ rods. If only the rods are activated, you don’t see color, just shades of gray.
Your past visual experiences with objects also influence your perception of color. This phenomenon is known as color constancy. Color constancy ensures that the perceived color of an object stays about the same when seen in different conditions. For example, if you looked at a lemon under a red light, you likely would still perceive the lemon to be yellow.
Color Vision Anomalies
Color blindness can occur when one or more of the cone types are not functioning as expected. Cones can be absent, nonfunctioning or detect a different color than normal. Red-green color blindness is the most common, followed by blue-yellow color blindness. Men are more likely to have color blindness than women. Scientists are currently developing new treatments for color blindness.
Researchers estimate that up to 12 percent of females have four cone types in their retinas, rather than three. These individuals have the potential to perceive 100 times more colors than the rest of us.
Many birds, insects and fish have four types of cones. With their different cones, they can see ultraviolet light. Ultraviolet light has wavelengths shorter than what the human eye can see. Other animals, such as dogs, have fewer types and numbers of cones, so they may see fewer colors than humans do.